Toxoplasmosis in dogs

Clinical information:

Toxoplasmosis is a zoonosis which occurs worldwide and is caused by the sporozoon Toxoplasma gondii. All warm-blooded animals can become infected with Toxoplasma (T.) gondii. The asexual reproduction of T. gondii can take place in the most diverse tissues of the warm-blooded intermediate hosts. The only final hosts are cats or other felidae, in whose intestine the sexual reproduction takes place. This leads to the formation of oocysts which are secreted into the environment with the cat’s faeces.

Dogs mostly become infected by ingesting raw meat containing tissue cysts or oocyst-containing cat faeces. Moreover, it was shown in dogs that the parasites can also be transmitted via sperm or transplacentally. T. gondii infections in dogs proceed asymptomatically in most cases. However, a severe clinical toxoplasmosis may develop particularly in young dogs, which may lead to death. Depending on the affected tissue, different clinical symptoms appear, such as fever, hepatitis or pneumonia. T. gondii parasites often attack the brain and lead to CNS symptoms.

In addition to its clinical relevance, canine toxoplasmosis is an important zoonosis. Humans are generally infected perorally by ingestion of water or food contaminated with oocysts or from meat products (the raw meat of infected animals contains cysts with viable trophozoites). As final hosts, cats secrete oocysts with their faeces and thus spread them in the environment. However, also dogs can be mechanical vectors, since they tend to ingest cat faeces. Oocysts ingested in this way may pass through the dog’s intestine unchanged and be secreted into the environment. Also by rolling in cat faeces may the dog’s fur become contaminated with oocysts, which are then passed on. The parasites become infectious after 2 to 4 days in the environment and can perorally infect humans or warm-blooded animals.

Diagnostics:

Owing to the variety of possible symptoms, the clinical picture is not very characteristic. In the case of suspected toxoplasmosis, detection of specific antibodies in serum or plasma by means of IIFT, ELISA or agglutination assay is the method of choice for confirming an infection. A positive IgG antibody result indicates an infection. Since the cysts, and therefore also the IgG antibodies persist lifelong, an acute infection can only be diagnosed by detection of IgM antibodies or by a fourfold increase in the IgG titer in a follow-up sample taken after 2 to 4 weeks. IgM antibodies are generally detectable 2 to 16 weeks after infection. Specific IgG, in contrast, is exhibited 3 to 4 weeks after infection and probably persists lifelong.